Organometallic compounds of Group III-A elements of the Periodic Table, and particularly the lower alkyl compounds of these elements, are extensively used to deposit compounds of their constituent elements on substrates by chemical vapor deposition. For example, gallium arsenide semiconductor layers have been deposited on substrates by combining the vapors of a gallium source such as trimethylgallium with an arsenic source such as arsine at an elevated temperature in the presence of a suitable substrate. Similar processes are used to form other III-V compounds, for example, indium phosphide from trimethylindium and phosphine. The Group III-A compounds are preferably supplied as liquids from a bubbler, where they are evaporated in a stream of carrier gas for delivery to the deposition chamber. For this mode of delivery, therefore, it is desirable that the organometallic sources of Group III-A compounds be volatile and be liquid at some temperature between roughly 0.degree. Celsius and 150.degree. Celsius.
The Group III-A source compounds for chemical vapor deposition, particularly for formation of III-V compounds, are required to be exceptionally pure to produce coatings of the grade necessary for electronic and other demanding applications. When the compounds are delivered as liquids from a bubbler, nonvolatile impurities are not especially significant because they are not evaporated in the bubbler apparatus, and thus are not transported to the substrate. Volatile impurities, however, are carried into the deposition chamber, and thus must be minimized in chemical vapor deposition source compounds. Even a few parts per million of volatile impurities can have a significant effect on the properties of the deposited film. See for example Jones, et al., "Analysis of High Purity Metalorganics by ICP Emission Spectrometry," Journal of Crystal Growth, Vol. 77, pp. 47-54 (1986), especially page 47, column 1. (This article is not admitted to be competent prior art.)
One aggravating factor is that many interfering solvents and organometallic compounds are volatile, and thus are difficult to separate from the volatile Group III-A sources by physical means. For example, the Jones, et al. article cited above indicates the difficulty of removing volatile microimpurities from Group III-A alkyl compounds by distillation. One specific example of organic contamination of a Group III-A alkyl compound is the presence of complexed ether in trialkylindium compounds, which are prepared in an ether solvent. The ether is tightly complexed and thus inseparable. Prior attempts to separate the complexed ether have involved high temperature, repeated distillations which waste most of the desired product and do not remove all the complexed ether. A specific example of organometallic contamination is the presence of alkyl silanes such as tetramethyl silane or Group II-B alkyls such as dimethylzinc.
The previously cited Jones article teaches that Group III-A organometallic compounds can be separated from organometallic and organic impurities by forming a nonvolatile adduct of the desired organometallic compound with 1,2-bis(diphenylphosphino) ethane. Materials which do not form such adducts, including ether, other organic impurities, and organometallic compounds of tin, silicon, zinc, etc. can then be evaporated and removed, since they are volatile and the adduct is not. The adduct is decomposed to release the desired volatile organometallic compound. The desired compound is then distilled away from the nonvolatile 1,2-bis (diphenylphosphino) ethane and recovered.
One significant problem with the Jones, et al. process is that the adduct must be decomposed by heating it. If the adduct decomposition temperature is high, it might approach or exceed the decomposition temperature of the desired compound. A second problem is that the Jones et al. adducting agent forms adducts with volatile organometallic compounds of elements other than Group III-A elements. These interfering compounds will be released when the adduct is decomposed unless their adducts decompose at a substantially higher temperature than the adducts of the desired organometallic compound. The Jones et al. process thus will not separate all the undesirable impurities.
Various commercial Group III-A alkyls are sold as containing less than about one part per million of impurities; such compounds are often referred to as having "six nines" or "six N" purity, indicating that they are at least 99.9999% pure. Stated another way, they are said to contain less than one part per million (ppm) of impurities. However, the common commercial standard of purity is the quantity of nonvolatile constituents present in the material. Since nonvolatile impurities are of less concern than volatile impurities, and since volatile impurities have not generally been measured, the claimed degree of purity of these compounds has not directly related to their utility for chemical vapor deposition of films.
Volatile impurities (other than solvents) can be determined using inductively coupled plasma atomic emission spectroscopy analysis, taking special precautions to retain volatile impurities in the sample during analysis. (See U.S. Pat. No. 4,688,935, issued Aug. 25, 1987 Barnes et al. and hereby incorporated herein by reference.) Volatile impurities (including solvents) are also detectable by mass spectroscopy, again providing that the impurities remain in the sample during analysis. Such analysis has demonstrated that commercial materials have not been of optimum purity. It is believed to be common for materials having "six nines" purity with respect to nonvolatile constituents to have "five nines" or less purity with respect to volatile constituents such as silicon alkyls and (for gallium compounds) zinc alkyls.